Pentolite-dipped cap wells in seismic cans



Julyl6, 1963 a 3,097.01

PENTOLITE D IPPED CAP WELLS IN SEISMIC CANS G. L. GRIFFITH .ETAL

Filed July 22, 19 60 FIG. 1

FIG. '2

Inv en 1ors George L. Griffith George A. Lyte their Attor e 3,097,601 PENTOLETE-DWPED CAP WELLS EN SEISMIC CANS George L. Griflith, Coopershurg, and George A. Lyte, Bethlehem, Pa., assignors to Trojan Powder- Company,

New York, N.Y., a corporation of New York Filed July 22, 1960, Ser. No. 44,789 4 Claims. (Cl. 102-23) This invention relates to explosives of the type which lose sensitivity when subjected to undue pressure. Where a train of cans connected for land seismic work is lowered into a bore hole and encounters a hydrostatic head of several hundred feet, there is a densification of the material in the cans with consequent loss of sensitivity to such an extent that the explosion initiating cap in the primer can of the train may not have sufficient power to detonate the train. The principal object of the present invention is to provide a method of counteracting such loss of sensitivity in a simple, inexpensive manner.

A further object of the invention is to provide a primer can having on its cap well outer surface a substance which may be varied in weight, viscosity, and grain size to secure successful firing of the primer can with a standard cap under any one of the various pressures which may be expected. A still further object of the invention is to provide a molten pentolite bath in which the cap well of an empty primer can may be dipped so that the pentolite coating may be smooth, highly adherent, free of brittleness, and of suitable sensitivity.

These desirable features are secured by control of the viscosity of the melt by varying the temperature, as well as selecting the appropriate grain size of the pentaerythritol tetranitrate used in about equal parts with the TNT to form the pentolite, and by altering the speed of the dipping mechanism to change the thickness of the applied coating and therefore its weight.

In the drawings:

FIG. 1 is a vertical section through a portion of a train of cans in a bore hole showing a primer can of our invention between two cans of seismic explosive.

FIG. 2 is a vertical section showing an empty primer can lowered into a dipping cup of molten pentolite.

In land seismic work it is standard practice to use a train of connected cans of explosive 2 or 2 /2" in diameter each weighing about a pound when loaded. Such a train, usually about 20 feet in length but sometimes twice that long or more, is loaded into a bore hole with a similar sized can of priming material screwed to one end of the train and the train is initiated by a cap in a well provided for that purpose in the primer can. Such cap wells are not supplied in the cans of seismic explosive since such explosive is normally too insensitive to be fired with a cap. As previously stated, under certain conditions the cans may be subjected to a hydrostatic head of several hundred feet and the pressure thus applied causes sorne compression of the can with consequent densification of the material therein to such an extent as to cause it to lose sensitivity. This is especially true of certain types of booster materials which become so easily desensitized by compression that the caps are no longer able to provide sufficient initiating power. It has now been found that such loss of sensitivity may be overcome quite simply by placing a small amount of pentolite on the cap Well of each primer can, usually one for each ten seismic explosive cans, before loading the primer can.

FIG. 1 shows a portion of a train including a primer or booster can between an upper can 11 of seismic explosive and a similar lower can 12, these cans being secured together in any desired manner, here shown as pro- 3,097,601 Patented July 16, 1963 vided by an externally threaded extension 14 at the bottom of each can screwed firmly into a similar but slightly larger internally threaded extension 15 at the top of the next lower can. The primer can upper extension corresponding to 15 is numbered 16 to distinguish it as it is centrally extended to form an elongated cylindrical well 17 of a diameter to receive the usual explosion initiating cap (not shown), the indentation 18 serving to provide a friction grip on the cap to resist its movement out of the well. The well is a container for the explosion initiating cap. The pentolite coating on the outside of the well 17 is indicated at 20 and while we prefer for optimum conditions that the coating shall cover as little "as one-third of the length of the tubular well it may cover more but should not get into the indentation 18. The bore hole 22 is about a half-inch larger in diameter than the cans.

In FIG. 2 there is shown a pentolite reservoir 25 connected by a pipe 26 to a dipping cup 27 of from 1'' to 1%" in diameter, the whole system, including the reservoir inlet and drain (not shown) being steam jacketed as at 28 to permit holding the contents of the dipping cup accurately at the temperature desired. The mechanism for lowering an unfilled primer can with its well passing into the dipping cup is conventional and forms no part of the invention. The primer can 30 which being empty has no bottom closure piece is lowered vertically around the steam jacket 28 of the dipping cup so that the desired third of the length of the tubular well is momentarily immersed in the pentolite. The liquid level of the molten height by suitable well-known means.

We'have found that the minimum effective coating to' be placed on the can well outer wall weighs one gram.

For optimum results from 1 /2 to 2%. grams should be used. While a coating much exceeding the latter amount may be used, it might be noted that the improvement when exceeding 2 /2 grams is at a low rate, compared with the distinct improvement secured by using a two gram coating, for example, rather than using a half-gram less.

Pentolite becomes molten at about C. and can be held at 100 C. for long periods of time without noticeable decomposition. The amount of pentolite deposited on the cap well may be controlled to a certain extent by altering the temperature of the molten pentolite. However, when the ordinary mill pentaerythritol tetranitr-ate is used in the pentolite the viscosity of the latter is so low at the temperatures cited that a satisfactory coating on the cap Well is difiicult to obtain. Redipping can be used as a resort when insufficient material is deposited on the first dip, but this is an added operation and expense, and experience shows that on a redip we usually melt off a large portion of the first dip so that the amount of pentolite remaining on the cap well still may be unsatisfactory even though the equipment is adjusted to accomplish the dipping as rapidly as is commensurate with avoidance of splashing.

We have found that we can control the viscosity of the melt at such a point that it will always give a satisfactory deposit at -95 C. and usually will do so at 82- C. by use of a selected grain size of pentaerythritol tetranitrate in the pentolite, the pentolite used usually being a 50:50 mixture of pentaerythritol tetranitrate and TNT in the first nine examples cited below, although other proportions have been used quite satisfactorily. Such proportions run from 4060% pentaerythritol tetranitrate and 6040% TNT, as in Examples 10 and 11.

Pentaerythritol tetranitrate of a grain size larger than that which will pass through a 60-mesh sieve is not generally satisfactory. A smooth cast coating is not obtained with coarser grain sizes, and the finer material tends to have somewhat better sensitivity. When the pentaerythritol tetranit-rate is coarser there is a tendency to brittleness, which causes a portion of the coating to flake off during subsequent loading operations.

Example 1 Pentaerythritol tetranitrate of the following grain size was.used:'

Percent +80 mesh 34 80+120 26 -120+200 19 -200+230 3 230 18 50 parts of the above-noted material and 50 parts of a standard grade TNT were mixed together to form the pentolite and the mass heated to 90 C. This provided a smooth coating when the cap wells were dipped therein. Theamount adhering to the well averaged 1.75 grams with a high of about 2 grams and a low of about 1.5 grams at the temperature cited. If an increased weight of the material is desired on the well an increased amount of the 230 mesh size pentaerythritol tetranitrate is used in the mixture as indicated below.

Example 2 A 50:50 pentolite was prepared as in Example 1 using pentaerythritol tetranitrate with the following screen analysis:

Percent +80 mesh 27 -80+120 20 -120+200 15 200+230 5 230 33 This mixture provided a smooth coating of approximately 2.5 grams on the primer well in one dip and without splashing.

Example 3 A 50:50 pentolite mixture was prepared as in the preceding examples using pentaerythritol tetranitrate with the following screen analysis:

Percent +80 mesh 2O -80+120 12 -120+200 -200+230 6 --230 52 When the primer wells were dipped in this mixture at 90 C. and with the same timing, the coating never weighed less than 6 grams nor more than 7 grams and averaged approximately 6.5 grams. While such a large quantity can be applied in a single dip by using such large amounts of very fine pentaerythritol tetranitrate this is not a practical production mixture or method. The pentolite containing such large amounts of the pentaerythritol tetranitrate is very viscous and is difiicult to handle. Furthermore, the weight of coating using such a viscous mixture varied widely for rather minor temperature changes. That the use of more than 2 /2 grams of pentolite on a cap well gives only moderately added advantage is seen below.

Example 4 An examination of-the preceding three examples indicates that the pentaerythritol tetranitrate used in the pentolite for dipping purposes should have the following approximate screen analysis:

Percent mesh 25-35 -80+120 20-30 120+200 15-20 200+230 25 -230 20-35 All of the above material, of course, should pass a 60 mesh screen. In accordance with the above-noted indications, a 50:50 pentolite was made up using a pentaerythritol tetranitrate with the following screen analysis:

Percent +80 mesh 30 -80+120 28 120-|-200 17 200+230 4 230 21 When this mixture was used for dipping purposes at SS- C. the coating was smooth, adherent and fairly uniform in weight ranging from 1.8 to 2.1 grams per cap well.

Example 5 (Control Test) A number of cans with undipped cap wells were loaded with the priming material and sealed after which they were immersed for 24 hrs. at a static head of 50 it. When these cans were removed from the pressure chamber they showed signs of compression and each failed to fire with a standard No. 6 blasting cap. Several of these cans were opened and the contents examined, but no evidence was found of water leakage.

Example 6 The cap wells of a number of cans were dipped into the mixture of Example 1 maintained at 104 C. to give a coating which averaged 1.2 grams. These cans were then loaded as in the preceding example, sealed, and subjected to a 50 ft. hydrostatic head for 24 hrs. At the end of that time they were removed from the pressure cham 'ber and tested for detonation with standard No. 6 caps. Approximately 25% failures to detonate resulted. Other cans of this group, tested prior to immersion, had shown no failures even with a standard No. 5 cap.

Example 7 As in Example '6 but the pentolite was maintained at 98 C. to give a heavier coating, a portion of these cans which had an average coating of 1.5 grams was loaded and sealed in the usual way and subjected to a hydrostatic head of 50 ft. for 24 hrs. These cans showed no failures to detonate with a No. 6 standard cap. A second group of these cans was subjected to a hydrostatic head of 75 ft. for 24 hrs. after which 10% failures to detonate were noted with the standard No. 6 cap.

Example 8 When the mixture of Example #4 was used at 90 C., the average coating was 2.1 grams and the cans then fired after 24 hrs. at a hydrostatic head of 75 ft. without any failures but gave 20% failures with the standard No. 6 cap after 24 hrs. at a hydrostatic head of 100 ft. The cans which failed were examined for water penetration but none was detected.

Example 9 The mixture of Example 3 was used at the same tempenature as in Example 8 which gave an average coating of six grams of pentolite on the cap wells. The cans were then loaded, sealed, and subjected to a hydrostatic head of 100 feet for 24 hours and found to give 100% detonation thereafter with standard No. 6 caps.

Example A mixture of 40 parts of PETN having the following screen analysis was used:

and 60 parts of TNT. The cap wells were dipped at 90 C. and the average coating amounted to 2.2 grams. This material, because of its higher TNT content, is a little less sensitive than the 50:50 mixture. For that reason No. 8 standard caps were used with it to insure against misfires. The loaded sealed cans were subjected to a hydrostatic head of 75 feet for 24 hours. There were no failures.

When this material containing 60 parts TNT cooled, it formed longer TNT crystals than were detected with the 50:50 mixture, and the layer deposited on the cap well was somewhat more brittle so that it could be chipped ofi more readily. Because of the brittleness and lower sensitivity of the mixture prepared from 60% TNT and 40% pentaerythritol tetranitrate it is not preferred, although it meets the requirements of the invention.

Example 11 Another mixture containing 40 parts of TNT and 60 parts of the 'PETN of Example 1 was used at 90 C. to give coatings averaging 2.1 grams. When tested as immediately above, the cans showed no failures to fire even with No. 4 standard caps. This was expected since the mixture containing 60% of PETN is more sensitive than the one containing 50% What we claim is:

1. The method of counteracting loss of sensitivity of explosives used in land seismic work due to undue pressures such as hydrostatic heads when a train of connected cans is used in a bore hole in which chosen spaced cans of seismic explosive of the train are replaced by primer cans containing priming material, each such primer can having a centrally positioned elongated cap well container extending within the can; which consists in coating, prior to filling the primer can with primer material, the outside of the wall of the primer cap well container with a smooth thin coat of even thickness of pentolite extending from the bottom of the well container upwardly, the total weight of the pentolite coating being between one and six grams and extending a substantial portion of the length of the cap well container.

2. The method of claim 1 in which the pentolite has a proportion of 60% of pentaeryt-hritol tetranitrate of a grain size smaller than will pass through a -mesh screen, and 60-40% of TNT.

3. The method of claim 2 in which the pentolite is 4050% TNT.

4. The method of claim 1 in which the coating is placed on the container by dipping the well into a pool of molten pentolite at a temperature of between and C. for sufiicient time to acquire a smooth coating of between 1 /2 and 2% grams, said pentolite consisting of roughly equal parts TNT and pent aerythritol tetranitrate, the latter of a grain size to pass a 60-mesh screen and of approximately the following screen analysis:

Percent +80 mesh 25-35 80 to mesh 20-30 120 to 200 mesh 15-20 200 to 230 mesh 2-5 230 mesh 20-35 References Cited in the file of this patent UNITED STATES PATENTS 2,063,601 Hu-mmel Dec. 8, 1936 2,235,009 Campbell Mar. 18, 1941 2,371,879 Davis et a1. Mar. 20, 1945 2,395,341 McCurdy Feb. 19, 1946 2,407,805 Wyler Sept. 17, 1946 2,709,407 Lowe May 31, 1955 2,913,982 Hayes Nov. 24, 1959 

1. THE METHOD OF COUNTERACTING LOSS OF SENSITIVITY OF EXPLOSIVES USED IN LAND SEISMIC WORK DUE TO UNDUE PRESSURES SUCH AS HYDROSTATIC HEADS WHEN A TRAIN OF CONNECTED CANS IS USED IN A BORE HOLE IN WHICH CHOSEN SPACED CANS OF SEISMIC EXPOLOSIVE OF THE TRAIN ARE REPLACED BY PRIMER CANS CONTAINING PRIMING MATERIAL, EACH SUCH PRIMER CAN HAVING A CENTRALLY POSITIONED ELONGATED CAP WELL CONTAINER EXTENDING WITHIN THE CAN; WHICH CONSISTS IN COATING, PRIOR TO FILLING THE PRIMER CAN WITH PRIMER MATERIAL, THE OUTSIDE OF THE WALL OF THE PRIMER CAP WELL CONTAINER WITH A SMOOTH THIN COAT OF EVEN THICKNESS OF PENTOLITE EXTENDING FROM THE BOTTOM OF THE WELL CONTAINER UPWARDLY, THE TOTAL WEIGHT OF THE PENTOLITE COATING BEING BETWEEN ONE 